Vanadium oxide, which shows semiconductor-like resistance changes, is used as a material for infrared sensors. The absolute value of the temperature coefficient of resistance of vanadium oxide at room temperature is about 2%/° C., and the value becomes smaller as the temperature becomes higher (Patent Document 1). In order to develop compact and high-precision infrared sensors, a material having a large absolute value of the temperature coefficient of resistance is required. Crystallized vanadium oxide undergoes metal-insulator transition at around 68° C., and its electric and optical properties are drastically changed in accordance with this transition. Thus, crystallized vanadium oxide is expected to be applied to infrared sensors which utilize a large change in electric resistance.
However, synthesis of crystallized vanadium oxide generally includes a heating step at 400° C. or more, and when vanadium oxide crystals are synthesized on a substrate containing polyimide resin, the polyimide resin is subjected to pyrolysis. To overcome this problem, Patent Document 2 discloses that a solution of a vanadium organic compound is applied onto a support, dried, and then irradiated with laser light having a wavelength of 400 nm or less in two stages to decompose the vanadium organic compound to thereby enable production of a crystallized vanadium oxide thin film for infrared sensors at a low temperature. The vanadium oxide thus crystallized has metal-insulator transition accompanied by structural phase transition against temperature changes and thus shows a hysteresis of resistivity changes due to temperature rising/falling. In application to infrared sensors, materials showing a hysteresis of resistivity changes due to temperature rising/falling are not preferable.
To overcome this problem, Patent Document 3 discloses that a thin film including an amorphous layer and a crystalline layer is produced by irradiation of laser having a wavelength of 400 nm or less. This thin film is a vanadium oxide resistor film used in infrared sensors and shows metal-insulator transition having substantially no hysteresis. However, this vanadium oxide resistor film has a large absolute value of the temperature coefficient of resistance only around room temperature. Terahertz sensors employing the current infrared sensor device technology have been developed (Patent Document 4). In order to improve the sensitivity of such sensors, a material which shows substantially no hysteresis of resistivity changes and has a large absolute value of the temperature coefficient of resistance has become required.
In order to solve the above problem, control of the metal-insulator transition temperature is required. It has been reported so far that doping vanadium oxide with a metal changes the metal-insulator transition temperature. In a VO2 film doped with Ti, a shift of the metal-insulator transition temperature toward high temperatures and broadening of the metal-insulator transition are observed in accordance with increase in the amount of Ti doped, and simultaneously, reduction in the hysteresis width has been observed (Non-Patent Document 1), but a problem of the high resistivity exists. In vanadium oxide doped only with W, a shift of the metal-insulator transition temperature toward low temperatures is observed, and reduction in the hysteresis width has been observed (Non-Patent Document 2).
In contrast, an oxide thin film for bolometers is known which satisfies 1.5≤x≤2.0 when vanadium oxide is represented by VOx and in which a portion of V is replaced by other metal M, wherein M comprises at least one of chromium (Cr), aluminum (Al), iron (Fe), manganese (Mn), niobium (Nb), tantalum (Ta), and titanium (Ti) (Patent Document 5). In the case of doping with these metals, the temperature coefficient of resistance at room temperature at which the absolute value is maximized is —4.15%/° C. in the case of doping with manganese. However, the absolute value of the temperature coefficient of resistance cannot be expected to increase at a temperature higher than room temperature because temperature-dependent properties of the resistance of vanadium oxide in which a portion of vanadium ions has been replaced by other element show a large slope at 0° C. to 20° C.
Additionally, in case of application of a certain material to infrared sensors, control of the resistance value is also important simultaneously as the temperature coefficient of resistance of this material. The resistance value of the bolometer material at room temperature is desirably about 5 kΩ to 100 kΩ, for example. When the thickness of the bolometer thin film is set to 0.05 μm to 1 μm, the resistivity required from the bolometer material is desirably about 0.025 Ωcm to 10 Ωcm (Patent Document 5). As above described, materials which show substantially no hysteresis of resistivity changes due to temperature rising/falling have a resistivity of 1 Ωcm or less when the thickness of the bolometer thin film is 100 nm, show semiconductor-like resistance changes over a wide temperature range, and have an absolute value of the temperature coefficient of resistance larger than that of vanadium oxide have not been reported.